The star around which the Earth revolves, and the
gravitational center of the Solar System;
it lies about 270,000 times closer to us than the next nearest star. The
Sun is a solitary, yellow dwarf star of spectral type G2 (see G
star) that has been on the main sequence for about 4.6 billion years.

The Sun consists largely of hydrogen (71%
by mass) and helium (27%), with much smaller
amounts of heavier elements. It puts out 400 trillion trillion watts of
energy, produced by the fusion of hydrogen
to helium by the carbon-nitrogen cycle in
its core. The Sun is 109 times wider than the Earth. It spins on its axis
with a period that varies from 25 days at the equator to 33.5 days near
the poles.

Sun-like stars and solar stability

Since the Sun is unique in having a known (to humans!) habitable planet,
it is natural that scientists first turn to stars similar to the Sun (see FGK stars and target
stars) in their search for extraterrestrial life. Superficially, there
are many such stars, including a handful that are less than 20 light-years
away (see Sunlike stars). During this decade
and beyond, attention will be focused on trying to detect Earthlike planets
orbiting within the habitable zones of such
solar look-alikes. However, it may be that in at least one respect the Sun
is abnormal. A consensus is emerging that our star is exceptionally stable.
Although like all stars it sends out flares from time to time, these tend
to be on a very modest scale by stellar norms. What is still unknown is why the Sun is so well-behaved, and whether we just happen to be
enjoying a ranquil phase in its career.

An almost-perfect
sphere

In 2012, a team of scientists led by Jeffrey Kuhn, of the University of
Hawaii, announced the first most precise measurement ever made of the Sun's oblateness – the extent to which
it bulges out at the equator.1 Surprisingly, it turns out, the
Sun is an almost perfect sphere. Despite being 1.4 million kilometers across,
its equatorial diameter is a mere 10 kilometers more than its diameter from
pole to pole. If the Sun were scaled down to the size of a beachball, the
difference between its equatorial and polar diameters would be less than
the width of a human hair. Given that the Sun spins on it axis, once every
28 days, and is made of (ionized) gas, this result implies that effects,
such as magnetism or turbulence, play a greater effect in determining the
overall shape of the Sun than had been expected.